1. Field of the Invention
The present invention generally relates to pulse tube cryogenic coolers. More particularly, the present invention relates to a pulse tube cryogenic cooler wherein a heat exchanger is provided at an end part of a pulse tube.
2. Description of the Related Art
Generally, a pulse tube cryogenic cooler consisted of a pressure vibration generating device, a regenerator, a pulse tube, a phase control mechanism, and others. Such a pulse tube cryogenic cooler is quieter than a Gifford McMahon (GM) cryogenic cooler or a Stirling type cryogenic cooler. Therefore, application of the pulse tube cryogenic cooler as a cooling device of various test or analyzing devices such as an electron microscope or a Nuclear Magnetic Resonance (NMR) apparatus has been expected.
FIG. 1 is a structural view of a related art double inlet type pulse tube cryogenic cooler.
Referring to FIG. 1, a helium compressor 1, a high pressure valve 3a and a low pressure valve 3b form a pressure vibration generating device. The high pressure valve 3a is provided at an output side of high pressure gas of the helium compressor 1. The low pressure valve 3b is provided at a gas receiving side of the helium compressor 1. This pressure vibration generating device is connected to a high temperature part 2a of a regenerator 2.
The high pressure valve 3a and the low pressure valve 3b are switched at a designated cycle. Therefore, helium gas having a high pressure and generated by the helium compressor 1 is supplied to the regenerator 2 at the designated cycle.
A housing 32 made of stainless steel is provided at upper ends of the regenerator 2 and the pulse tube 6.
In addition, lower ends of the regenerator 2 and the pulse tube 6 are connected to each other by a connection path 4. More specifically, a heat exchanger 5b is provided at the lower end of the pulse tube 6. This heat exchanger 5b and a low temperature part 2b of the regenerator 2 are connected by the connection path 4.
Furthermore, a buffer tank 8 is connected to a high temperature end, namely an upper end, of the pulse tube 6 via a heat exchanger 5a and an orifice 7a. 
In addition, a bypass path 9 is provided between a pipe connecting the pressure vibration generating device and the regenerator 2 and a pipe connecting the pulse tube 6 and the buffer tank 8. An orifice 7b is provided in this bypass path 9.
The regenerator 2 is filled with a cold storage material such as wire gauze made of copper or stainless. A punching plate made of aluminum or the like or a copper mesh 10 is laminated inside the heat exchangers 5a and 5b as a heat exchanging member. A numerical reference 11 denotes a rectifier.
In the above-discussed pulse tube cryogenic cooler, when the high pressure valve 3a is opened and the low pressure valve 3b is closed so that an operation mode is started, helium gas compressed by the compressor 1 and having high pressure flows into the regenerator 2.
The helium gas flowing into the regenerator 2 is cooled by the cold storage material provided in the regenerator 2 so that the temperature of the helium gas is decreased. The helium gas flows from the low temperature part 2b of the regenerator 2 to the heat exchanger 5b via the connection path 4 so as to be further cooled and flows into the low temperature side of the pulse tube 6.
Gas having low pressure and already existing in the pulse tube 6 is compressed by the operation gas newly flowing in. Therefore, pressure in the pulse tube 6 becomes higher than pressure in the buffer tank 8. Because of this, the operation gas in the pulse tube 6 flows into the buffer tank 8 via the orifice 7a. 
When the high pressure valve 3a is closed and the low pressure valve 3b is opened, the operation gas in the pulse tube 6 flows into the low temperature part 2b of the regenerator 2. The operation gas passes an inside of the regenerator 2 and flows from the high temperature part 2a to the compressor 1 via the low pressure valve 3b. 
As discussed above, the high temperature end of the pulse tube 3 and the high temperature part 2a of the regenerator 2 are connected by the bypass path 9 having the orifice 7b. Because of this, the phase of pressure change and the phase of volume change of the operation gas occur with a constant phase difference.
Due to the phase difference, a cold state is generated as the operation gas is expanded at the low temperature end of the pulse tube 6. By repeating the above-discussed steps, the pulse tube cryogenic cooler works as a cryogenic cooler. In the above-discussed double orifice type pulse tube cryogenic cooler, the phase difference can be adjusted by adjusting the orifice 7b provided in the bypass path 9.
In addition, the heat exchanger 5a is provided at the upper end of the pulse tube 6 and the heat exchanger 5b is provided at the lower end of the pulse tube 6 in order to improve cooling efficiency and increase the heat transfer property.
More specifically, as shown in FIG. 2 in an enlarged manner, the mesh 10 made of aluminum or copper is laminated in the heat exchanger 5a provided at the upper end of the pulse tube 6 and positioned in the housing 32. Here, FIG. 2 is a cross-sectional view showing the heat exchanger provided in the related art pulse tube cryogenic cooler.
On the other hand, a structure shown in FIG. 3 and discussed in Japanese Laid-Open Patent Application Publication No. 2003-148826 is known as a heat exchanger having another structure. In the structure disclosed in Japanese Laid-Open Patent Application Publication No. 2003-148826, a laminated wire mesh and a porous plate are combined or a first porous plate, a first laminated wire mesh, a second porous plate and a second laminated wire mesh are provided in a parallel manner.
Here, in the related art pulse tube cryogenic cooler shown in FIG. 1 and FIG. 2, heat conductivity of the heat exchanger 5a between the pulse tube 6 and the housing 32 is discussed.
In the heat exchanger of the related art pulse tube cryogenic cooler, the punching plates or mesh members having the same roughness of meshes are laminated or plural porous plates and the laminated wire mesh having the same roughness of meshes are laminated. Therefore, when helium gas follows from the housing 32 to the pulse tube 6, heat exchanging between the flowing helium gas and the housing 32 cannot be made well. Because of this, helium gas having high temperature flows in the pulse tube 6 so that the cooling efficiency of the pulse tube 6 is decreased due to heat entry.
In addition, when helium gas flows from the pulse tube 6 to the housing 32, it is necessary to transfer heat to the helium gas from the housing 32 from the view point of keeping characteristics of the orifice 7a and the buffer tank 8. However, this heat exchanging cannot be made well due to the same reason as discussed above.